Identify the correct S.I. unit for normal stress (and shear stress) used in solid mechanics and materials engineering.

Introduction:
Stress is force per unit area. In the International System of Units (S.I.), it must be expressed using S.I. coherent units to ensure consistency in calculations, reporting, and code compliance.

Given Data / Assumptions:

• Stress σ or τ defined as Force / Area.
• Force in Newtons (N); area in square metres (m^2).
• Use of S.I. coherent units.

Concept / Approach:
The S.I. coherent unit of stress is N/m^2, named the Pascal (Pa). Engineering practice often uses derived multiples such as MPa where 1 MPa = 1 N/mm^2 = 10^6 N/m^2. However, the base S.I. expression remains N/m^2 (Pa).

Step-by-Step Solution:
1) Formula: stress = Force / Area.2) Substitute S.I. units: N / m^2 = N/m^2.3) Recognize naming: 1 Pa = 1 N/m^2; 1 MPa = 10^6 Pa = 1 N/mm^2.4) Conclude the correct S.I. unit is N/m^2 (Pa).

Verification / Alternative check:
Dimensional analysis confirms: [N] = kg·m/s^2; dividing by m^2 gives kg/(m·s^2), consistent with pressure/stress units.

Why Other Options Are Wrong:
N/mm^2: acceptable engineering unit (MPa) but not the base S.I. expression.

kN/mm^2: equals GPa order (10^9 N/m^2) and is rarely used; not the base S.I. unit.

Any one of these: incorrect because only N/m^2 (Pa) is the S.I. coherent unit.

Common Pitfalls:
Mixing base S.I. units with convenient engineering multiples; misreporting units in calculations.

Final Answer:
N/m2